This maybe very elementary, but are you getting absolutely all of the sizing lubricant off your cartridges?
I can flatten primers, get extractor marks, hard bolt lift,all kinds of evil stuff by just schwabbing some Case lube on a cartridge I've been loading for 15 years, and shooting it. Same applies to cleaning your gun and not drying the chamber real good. You know how a teeny bit of die wax will occasionally dent a shoulder? The same only more for slipping the brass down the chamber when it's fired.
Sierra tech support helped me with that when I first started reloading.
I can flatten primers, get extractor marks, hard bolt lift,all kinds of evil stuff by just schwabbing some Case lube on a cartridge I've been loading for 15 years, and shooting it. Same applies to cleaning your gun and not drying the chamber real good. You know how a teeny bit of die wax will occasionally dent a shoulder? The same only more for slipping the brass down the chamber when it's fired.
Sierra tech support helped me with that when I first started reloading.
